LT3092_15 Datasheet, PDF (11/22 Pages) Linear Technology – 200mA 2-Terminal Programmable Current Source

LT3092
APPLICATIONS INFORMATION
capacitor on the lower impedance output, and the same
restrictions do not apply. Capacitors in the range of 0.1μF
to 1μF usually provide sufficient bypassing on the input,
and the value of input capacitance may be increased
without limit.
If an application uses GND referred capacitors on the input
or output (particularly the input), pay attention to the length
of the lines powering and returning ground from the circuit.
In the case where long power supply and return lines are
coupled with low ESR input capacitors, application-specific
voltage spikes, oscillations and reliability concerns may
be seen. This is not an issue with LT3092 stability, but
rather the low ESR capacitor forming a high-Q resonant
tank circuit with the inductance of the input wires. Adding
series resistance with the input of the LT3092, or with the
input capacitor, often solves this. Resistor values of 0.1Ω
to 1Ω are often sufficient to dampen this resonance.
Give extra consideration to the use of ceramic capacitors.
Ceramic capacitors are manufactured with a variety of di-
electrics, each with different behavior across temperature
and applied voltage. The most common dielectrics used
are specified with EIA temperature characteristic codes of
Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are
good for providing high capacitances in a small package,
but they tend to have strong voltage and temperature
coefficients as shown in Figures 5 and 6. When used with
a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an
effective value as low as 1μF to 2μF for the DC bias voltage
applied and over the operating temperature range. The X5R
and X7R dielectrics result in more stable characteristics
and are more suitable for use as the output capacitor.
The X7R type has better stability across temperature,
while the X5R is less expensive and is available in higher
values. Care still must be exercised when using X5R and
X7R capacitors; the X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to DC
bias with X5R and X7R capacitors is better than Y5V and
Z5U capacitors, but can still be significant enough to drop
capacitor values below appropriate levels. Capacitor DC
bias characteristics tend to improve as component case
size increases, but expected capacitance at operating
voltage should be verified.
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress. In a
ceramic capacitor the stress can be induced by vibrations
in the system or thermal transients.
20
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
0
X5R
–20
–40
–60
Y5V
–80
–100
0 2 4 6 8 10 12 14 16
DC BIAS VOLTAGE (V)
3092 F05
Figure 5. Ceramic Capacitor DC Bias Characteristics
40
20
0
X5R
–20
–40
Y5V
–60
–80 BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
–100
–50 –25 0 25 50 75
TEMPERATURE (°C)
100 125
3092 F06
Figure 6. Ceramic Capacitor Temperature Characteristics
3092fb
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